Path: utzoo!attcan!uunet!mailrus!cs.utexas.edu!samsung!dali.cs.montana.edu!milton!gwplog From: gwplog@milton.u.washington.edu (Grant Petty) Newsgroups: sci.electronics Subject: Re: St Elmo's Fire (was Re: Lightning) Summary: Some lighting facts Keywords: Lightning, atmospheric electricity Message-ID: <5564@milton.u.washington.edu> Date: 26 Jul 90 18:02:52 GMT References: <1203@gold.GVG.TEK.COM> Organization: University of Washington, Seattle Lines: 26 Here are a couple of facts about lightning and atmospheric electricity, taken from "An Introduction to Atmospheric Physics (2nd Ed.)" by R.G. Fleagle and J.A. Businger. Average "fair weather" atmospheric current over entire earth: about 4 X 10^(-12) A sq. meter or 0.00001 Amps per square mile (positive current downward) Lightning strokes typically begin as a downward discharge of negative charge called a pilot leader, of roughly ~100 meters length. The pilot leader extends itself downward further as replenishing charges are conducting downward through the ionized path. Eventually the pilot leader is close enough to the earth's surface to create a large potential gradient exceeds the breakdown potential of the air in the vicinity of sharp objects. This induces a positive streamer to advance upward from the surface, where it meets the pilot leader at a height of 5-50 meters. When the two meet, a conducting path is formed between cloud and ground, and up this path rushes a wave of ionizing potential (called the return stroke) at a speed of about 100,000,000 meters/sec. This fully ionizes the channel, and the negative charge in the cloud base "rushes earthward through the brilliantly luminous channel and discharges roughly the lowest kilometer of the cloud." After a recovery time of about 1/20 sec., the potential redevelops, allowing a "dart leader" to reactivate the channel for another round of current exchange. The total number of strokes averages about three or four in the space of less than a second. Typical charge transferred in lightning strike: about 20 Coulombs